The present invention relates to a read circuit for a floppy disk drive (to be referred to as an FDD hereinafter).
FDDs are commonly used as external storage units for data processors such as computers and wordprocessors.
A conventional FDD causes a magnetic head to read a magnetic inversion pattern from a disk and causes a predetermined signal processor to digitize the read data and perform subsequent processing.
FIG. 1 shows a conventional signal processor. In the signal processor, an output signal from a magnetic head is amplified by preamplifier 1. The high-frequency noise component of an output signal from preamplifier 1 is then eliminated by low-pass filter (LPF) 3. The output waveform of the signal from LPF 3 is differentiated by differentiator 5. The zero-crossing points of the output signal from differentiator 5 are detected by zero-cross comparator 7, and digitization is performed. Noise is removed from the output signal of comparator 7 by time-domain filter 9.
FIG. 2 is a circuit diagram showing a detailed arrangement of differentiator 5 in the signal processor of FIG. 1. In differentiator 5, the output signal from the magnetic head is supplied to the bases of transistors Q1 and Q2, which are differentially connected. Differentiated waveforms are extracted from the collectors of transistors Q1 and Q2. Filter Z (hereinafter called a "differential filter"), for determining differential characteristics, is connected between the emitters of transistors Q1 and Q2.
FIGS. 3A through 3C show the frequency characteristics of filter Z when it is constituted of (i) only a capacitor, (ii) a series circuit of a capacitor and a resistor, and (iii) a series circuit of a capacitor, a resistor and a coil. Case (i) shows good differential characteristics but suffers from amplified high-frequency noise. Case (ii) offers increase in suppressed highfrequency noise. Case (iii) provides a decrease in high-frequency noise.
In the characteristics of case (iii), resonance frequency f0 and damping value .eta. are given: ##EQU1##
In the conventional FDD, the values of coil L, capacitor C and resistor R, constituting the differential filter for obtaining the characteristics in case (iii), are constant. Frequency f0 and damping constant .eta. of the differentiator are constant at the inner and outer sides of the disk.
In practice, however, the waveform of a signal read from the inner side of a disk differs greatly from that read from the outer side thereof.
FIGS. 4A and 4B show raw signals read from the inner and outer sides of a disk, respectively. As is apparent from FIGS. 4A and 4B, the waveform of the signal read from the outer side of the disk includes a larger number of harmonic components (especially the third harmonic component) than that of the signal read from the inner side of the disk.
FIG. 5 shows the differentiated waveforms of signals read from the inner and outer sides of a disk and passed through the conventional differentiator consisting of coil L, capacitor C and resistor R, the values of which are all fixed. In FIG. 5, the solid line represents the signal read from the inner side of the disk, and the dotted line represents the signal read from the outer side of the disk. As is apparent from FIG. 5, the gap of the differentiated waveform of signal A, read from the outer side of the disk, is smaller than that of the gap of the differentiated waveform of signal B, read from the inner side of the disk. When the gap of the differentiated waveform is decreased and noise is superposed thereon, pseudo zero-crossing points are generated. Comparator 7 then generates false data pulses, thus causing read errors.
In conventional FDDs, this problem is resolved by decreasing cut-off frequency fc of the LPF at the outer side of the disk (this technique is called a switch filter scheme), or by increasing write current Iw at the outer side of the disk.
These techniques are based on the fact that the output, resolution and read margin of data from the outer side of the disk are higher and larger than for data from the inner side of the disk. These techniques purposely degrade the resolution of the signal read from the outer side of the disk to decrease the third harmonic component thereof. Even if the read margin at the outer side of the disk is slightly decreased, performance is not substantially degraded.
The resolution and the read margin will now be described.
FIG. 6 is a graph showing the frequency characteristics which the magnetic head and the disk exhibit when data is read by the magnetic head from the disk. As is apparent from FIG. 6, when the density of data recorded on the disk is increased, the signal level is lowered. In a conventional FDD, the velocities of the inner and outer sides of the disk and the data transfer rates thereof are constant. The recording density at the inner side of the disk is, therefore, inevitably higher than at the outer side.
The signal level of the magnetic head is lower at the outer side of the disk than at the inner side. For this reason, the signal read from the inner side of the disk has lower resolution than the signal read from the outer side. As resolution decreases, peak shifting increases due to waveform interference.
The read margin is called "noise margin", and can be considered as the degree of occurrence of read errors with respect to variations of data pulses due to peak shifting by waveform interference, noise, and rotational variations of the motor.
As shown in FIG. 7, assume that the widths of the read gate pulses in data pulses are gradually decreased to find a limit beyond which read errors do not occur. The limit is raised when margin T is increased, thereby improving the performance of the read circuit.
According to the switch filter scheme in conventional FDDs, however, the LPF filter constant is changed. Hence, a large number of components, especially resistors and capacitors, are required, resulting in a complex arrangement.
In the conventional scheme for increasing the write current at the outer side of the disk, the write current must be large to obtain a sufficient effect.
Still another conventional technique to solve the above problems is disclosed in U.S. Pat. No. 4,244,008. U.S. Pat. No. 4,244,008 describes a read compensation circuit in a magnetic recording apparatus. According to this technique, the frequency characteristics of LPF 3 and the damping constant of differentiator 5 are changed. Harmonic components are increased to emphasize the waveform of the signal portion. As shown in FIG. 1 (prior art, or the U.S. patent), coils (L) 54a and 54b are selectively operated to change the L value, thereby changing the damping constant. However, when the coil value changes, the peak deviates from its normal position.
Recent FDDs use high-density recording media to increase the memory capacity. In these FDDs, a read circuit must read recorded data not only from a high-density magnetic recording medium but also from a low-density magnetic recording medium, assuming the two media are of the same shape. The rate of transferring data from the high-density magnetic recording medium is different from that of the low-density magnetic recording medium. There is a difference between the frequency bands of the signals from the two media. If the magnetic recording medium spins at a constant speed, and its recording density is doubled, the data transfer rate and, hence, the frequency bandwidth of the read signal are also doubled.
In this FDD, differentiator 3, provided in the read circuit, is adjusted such that differential filter characteristics correspond to high-density recording, providing a broadened signal frequency band.
When, in a read circuit with a differential filter arranged as described above, data is read from the low-density magnetic recording medium, high-frequency noise in the read signal is emphasized, thus decreasing the S/N ratio and read margin of the signal. Since a high-density recording head has a narrow read gap, resolution increases excessively when data is read from a low-density magnetic recording medium. The harmonic component also increases in the differentiated waveform of the read signal. As a result, error pulses are often mixed in with the read data pulses.
To solve the above problem, still another scheme has been proposed in which the cut-off frequency of the LPF is switched. However, a multi-stage filter arrangement is required, resulting in a complex circuit and a large number of components.